1. Field of the Invention
The present invention relates to a semiconductor controlled rectifier which is turned on by a gate signal.
2. Description of the Prior Art
In general, a semiconductor controlled rectifier which turns on upon application of a gate signal to a gate electrode comprises a semiconductor substrate having four continuous layers having alternatively different conduction types of PNPN, a pair of main electrodes respectively in ohmic contact with the outermost layers of the substrate, and a gate electrode in contact with the intermediate layer of the substrate. In such a semiconductor rectifier, if a gate voltage is applied between the gate electrode and one of the main electrodes in contact with the N-type outermost layer with a voltage being applied between the main electrodes so that the one main electrode is at a lower potential than that of the other main electrode is at a lower potential than that of the other main electrode in contact with the P-type outermost layer, a load current begins to flow between the main electrodes of the semiconductor controlled rectifier which has hereunto been in the non-conductive state. The switching of the semiconductor rectifier from the non-conducting state to the conductive state in this way is referred to as that the semiconductor controlled rectifier is turned on. The turn-on of the semiconductor controlled rectifier is performed in such a mechanism that a small area in the vicinity of the gate electrode is initially turned on by the gate current and thereafter the conduction area expands progressively with the elapse of time. Accordingly, if the current increasing rate di/dt is great at the time of the turn-on, the current density in the restricted conduction area near the gate electrode will become excessively large, resulting in the increase in temperature in that area so that the semiconductor controlled rectifier is likely to be thermally damaged or destructed.
There have been proposed various methods to increase the capability of a semiconductor controlled rectifier for the current increasing rate di/dt in an effort to protect the rectifier from the thermal destruction. For example, a semiconductor controlled rectifier is proposed in which a ring-shaped gate electrode is provided so that an initial conduction may occur along the whole periphery of one of the outermost layers. This rectifier has, however, a drawback that a remarkably large gate current is required for the turn-on. One of the most desirable conditions for the semiconductor controlled rectifier is that a large areas should become concuctive rapidly with a small gate current. As a rectifier which fulfills this condition, a semiconductor controlled rectifier of amplifying gate type or regenerative gate type is known.
In the case of a semiconductor controlled rectifier of amplifying gate type, a small region is formed at a location positioned between the gate electrode in contact with an intermediate layer and one of the outermost layers adjacent thereto, which region is of the same conduction type as that of the outermost layer and is electrically connected to the surface of the intermediate layer through an auxiliary electrode at the side remote from the gate electrode. With such a structure, the gate current from the gate electrode will initially turn on a first four-layer region whose one outer layer is said small region, and subsequently the main region of the semiconductor controlled rectifier (i.e. a second four-layer region whose one outer layer is said one outermost layer) is turned on by the load current flowing through the first four-layer region which then serves as the gate current for the second four-layer region. In this manner, a semiconductor controlled rectifier can be obtained in which the initial conduction can occur rapidly over a relatively large area with a small gate current. However, the rectifier device of the amplifying gate type as mentioned above has a drawback that the minimum value of the forward voltage applied between the main electrodes for turning on the second four-layer region of the semiconductor controlled rectifier will amount to a relatively great value. In more detail, in the semiconductor controlled rectifier of the amplifying gate type, the first four-layer region whose outer layer is said small region is turned to to cause the flow of the load current, as above described. Since this load current will flow into said one outermost layer across the intermediate layer from the auxiliary electrode spaced from the one outermost layer by a predetermined distance, the voltage applied between the main electrodes has to be increased by an amount corresponding to the voltage drop due to the resistance between the one outermost layer and the small region in order that the second four-layer region of the device may be turned on, and thus a higher voltage is required as compared with the case of turning on by the gate current the second four-layer region whose one outer layer is said one outermost layer. The voltage drop becomes greater, as the latching current (the minimum current allowing the forward current to continue to flow) of the first four-layer region is increased, that is, as the semiconductor controlled rectifier has a higher breakdown voltage. In other words, the minimum value of the forward voltage required between the main electrodes for turning on the rectifier device (this minimum voltage will be hereinafter referred to as finger voltage) becomes higher depending upon the increase in the voltage drop between the small region and the one outermost layer. As a result of that, when such semiconductor controlled rectifiers are directly connected in parallel with each other, imbalance in current appears remarkably. More specifically, there may happen such a case that one of the semiconductor controlled rectifiers connected in parallel will remain turned-off until the forward current of the other turned-on semiconductor controlled rectifier is increased so that the forward voltage thereof attains the finger voltage of the turned-off semiconductor controlled rectifier. Accordingly, when the semiconductor controlled rectifiers are connected in parallel with each other, the current imbalance will become greater, as the finger voltage becomes higher. For example, assume that semiconductor controlled rectifiers having respective finger voltages of 1.3 and 1.7 volts are connected directly in parallel with each other. If the gate signal is applied at the time when the voltage applied across the main electrodes exceeds a value greater than the finger voltage (1.7 volts), the difference in time of the above rectifiers being turned on is as small as 0.2 .mu. sec. On the contrary, when the gate signal is applied with the smaller forward voltage then the finger voltage (1.7 volts), the difference between the times when the rectifiers are turned on amounts to as great as 2 m sec.
Another disadvantage of the semiconductor controlled rectifier of the amplifying gate type can be seen in the fact that the rectifier may be turned on before the gate signal is applied, in the case where the increasing rate dv/dt of the applied voltage is relatively great or the rectifier is at a relatively high temperature. In the semiconductor controlled rectifier of the amplifying gate type, the small region and the auxiliary electrode are provided in the intermediate layer adjacent to the one outermost layer, as described hereinbefore. Accordingly, the area of the other intermediate layer which is not adjacent to said one outermost layer is larger than the former intermediate layer, as a result of which the displacement current and the reverse leakage current produced in the interior portion of the rectifier which is not covered by the one outermost layer (the portion which is not coincidently superposed on the one outermost layer, when projected in the laminated direction of the layers) is concentrated in the peripheral portion of the one outermost layer. Thus turn-on occurs in the peripheral portion of the one outermost layer before the gate signal is applied. Such a turn-on that occurs before the gate signal is applied (hereinafter referred to as erroneous turn-on) not only renders it impossible to control the semiconductor controlled rectifier, but also thermally destroys the semiconductor controlled rectifier itself. In brief, the rectifier device of the amplifying gate type will certainly provide an advantageous effect that the second four-layer region having the outer layer defined by the one outermost layer can be rapidly turned on over a wide area by initially turning on the first four-layer region having the outer layer defined by the small region. However, the rectifier has the drawback that, in the case where the portion of the second four-layer region with the outer layer thereof defined by the one outermost layer is initially turned on, the load current will be concentrated in such a turned-on portion to thereby bring about, possibly, the thermal destruction of the rectifier device, as above described. It will thus be appreciated that the semiconductor controlled rectifier may erroneously be turned on due to a displacement current or leakage current and thermally destroyed, when the increasing rate dv/dt of the applied voltage is high or the rectifier is at a high temperature.
On the other hand, the semiconductor controlled rectifier of the regenerative gate type is the one wherein one of the outermost layers is provided with a projection which partially projects toward a gate electrode without being connected to the main electrodes, and a given portion of the projection and a portion of an intermediate layer positioned oppositely adjacent to the one outermost layer are electrically connected to each other through an auxiliary electrode. In operation, the projection is first turned on by the gate current from the gate electrode, and the potential difference appearing between the portion of the projection contacted with the auxiliary electrode and the main electrode due to the load current allowed to flow by the turn-on is applied through the auxiliary electrode between the portion of the intermediate layer contacted to the auxiliary electrode and the main electrode to thereby turn on the portion of the one outermost layer located in opposition to the auxiliary electrode. The semiconductor controlled rectifier of this type also has the drawback that the minimum value of the forward voltage required between the main electrodes for turning on the rectifier, namely, the finger voltage is high as is in the case of the semiconductor controlled rectifier of amplifying gate type. In more detail, in the case of the semiconductor controlled rectifier of the regenerative gate type, the first four-layer region having an outer layer defined by the projection is initially turned on, whereby the resulting load current flows into the one outermost layer from the auxiliary electrode across the intermediate layer. When such semiconductor controlled rectifiers of the regenerative gate type are connected directly in parallel with each other, an imbalance in current will become remarkable, as is in the case of the amplifying gate type.